The present invention relates to projection exposure apparatus and methods used in fabrication of microdevices, for example, such as semiconductor integrated circuits, imaging devices including CCDs and the like, liquid crystal displays, or thin-film magnetic heads by the lithography technology, and to projection optical systems suitably applicable to such projection exposure apparatus. The present invention also relates to methods of fabricating the foregoing projection exposure apparatus and projection optical systems.
As the circuit patterns for such microdevices as the semiconductor integrated circuits and others are becoming finer and finer in recent years, the wavelengths of illumination light(radiation) for exposure (exposure light(radiation)) used in the exposure apparatus such as steppers and the like have been decreasing toward shorter wavelengths year after year. Namely, as the exposure light, KrF excimer laser light (wavelength: 248 nm) is going mainstream in place of the i-line (wavelength: 365 nm) of mercury lamps mainly used conventionally, and ArF excimer laser light of a much shorter wavelength (wavelength: 193 nm) is also nearing practical use. For the purpose of further decreasing the wavelength of the exposure light, there are also attempts to use halogen molecular lasers and others like the F2 laser (wavelength: 157 nm).
Although the aforementioned excimer lasers, halogen molecular lasers, etc. are available as light sources in the vacuum ultraviolet region of wavelengths not more than 200 nm, there are limits to practical band narrowing thereof.
Since limited materials transmit the emitted light in this vacuum ultraviolet region, available materials are limited for lens elements constituting the projection optical systems and transmittances of the limited materials are not so high, either. As matters now stand, the performance of antireflection coats provided on surfaces of the lens elements is not so high, as against those for longer wavelengths.
A first object of the present invention is to suppress chromatic aberration of the projection optical system and reduce loads on the light source.
A second object of the present invention is to correct chromatic aberration for the exposure light having some spectral width, by adding a single kind of glass material or a few color-correcting glass materials.
A third object of the present invention is to obtain an extremely fine microdevice circuit pattern while simplifying the structure of the projection optical system.
A fourth object of the present invention is to obtain an extremely fine microdevice circuit pattern without decrease in throughput.
For accomplishing the foregoing first or second object, a first projection optical system according to the present invention is a dioptric projection optical system for forming an image of a pattern on a first surface, onto a second surface by action of light-transmitting (radiation-transmitting) refractors, comprising: a front lens unit having a positive refracting power, located in an optical path between the first surface and the second surface; a rear lens unit having a positive refracting power, located in an optical path between the front lens unit and the second surface; and an aperture stop located in the vicinity of a rear focus position of the front lens unit; the projection optical system being telecentric on the first surface side and on the second surface side, wherein the following condition is satisfied:
0.065 less than f2/L less than 0.125,
where f2 is a focal length of the rear lens unit and L is a distance from the first surface to the second surface.
A first fabrication method of a projection optical system according to the present invention is a method of fabricating a dioptric projection optical system for forming an image of a pattern on a first surface, onto a second surface by action of radiation-transmitting refractors, comprising: a step of locating a front lens unit having a positive refracting power; a step of locating a rear lens unit having a positive refracting power, between the front lens unit and the second surface; and a step of locating an aperture stop between the front lens unit and the rear lens unit; wherein the front lens unit, the rear lens unit, and the aperture stop are located so that the projection optical system is telecentric on the first surface side and on the second surface side, and said method using the projection optical system satisfying the following condition;
0.065 less than f2/L less than 0.125,
where f2 is a focal length of the rear lens unit and L a distance from the first surface to the second surface.
For accomplishing the foregoing first or second object, a second projection optical system according to the present invention is a dioptric projection optical system for forming an image of a pattern on a first surface, onto a second surface by action of radiation-transmitting refractors, comprising three or more lenses having their respective refracting powers, wherein when three lenses are selected in order from the first surface side of the lenses having their respective refracting powers, at least one surface of the three lenses is of an aspheric shape having a negative refracting power.
For accomplishing the foregoing first or second object, a third projection optical system according to the present invention is a dioptric projection optical system for forming an image of a pattern on a first surface, onto a second surface by action of radiation-transmitting refractors, comprising a plurality of lenses having their respective refracting powers, wherein when two lenses are selected in order from the first surface of the lenses having their respective refracting powers, at least one surface of the two lenses is an aspheric surface, and wherein, where Ca is a local, principal curvature near a center of an optical axis of the aspheric surface and Cb is a local, principal curvature in the meridional direction of an extreme marginal region of a lens clear aperture diameter of the aspheric surface, the following condition holds if the aspheric surface has a negative refracting power:
Cb/Ca less than 0.7xe2x80x83xe2x80x83(b-1);
on the other hand, in the present invention, the following condition holds if the aspheric surface has a positive refracting power:
Cb/Ca greater than 1.6xe2x80x83xe2x80x83(b-2).
In this invention, the local, principal curvature Ca near the center of the optical axis of the aspheric surface and the local, principal curvature Cb is in the meridional direction of the extreme marginal region of the lens clear aperture diameter of the aspheric surface can be expressed as follows as an example. That is to say, the aspheric surface is expressed by the following equation (b-3):                                           Z            ⁡                          (              Y              )                                =                                                                      Y                  2                                /                r                                            1                +                                                      1                    -                                                                  (                                                  1                          +                          κ                                                )                                            ⁢                                                                        Y                          2                                                /                                                  r                          2                                                                                                                                          +                          AY              4                        +                          BY              6                        +                          CY              8                        +                          DY              10                        +                          EY              12                        +                          FY              14                                      ,                            (b-3)            
where Y is a height of the aspheric surface from the optical axis, z a distance along the direction of the optical axis from a tangent plane at the vertex of the aspheric surface to the aspheric surface, r a radius of curvature at the vertex, xcexa a conical coefficient, and A, B, C, D, E, and F aspheric coefficients.
With this expression, the local, principal curvatures Ca and Cb are given as follows.
Ca=1/rxe2x80x83xe2x80x83(b-4)
                    Cb        =                                                            ⅆ                2                            ⁢              z                        /                                          ⅆ                2                            ⁢              Y                                                          {                              1                +                                                      (                                                                  ⅆ                        z                                            /                                              ⅆ                        Y                                                              )                                    2                                            }                                      3              /              2                                                          (b-5)            
With increase in the numerical apertures of the projection optical systems and with increase in the size of the image field, there are increasing demands for minimization of distortion. In order to correct only distortion while suppressing influence on the other aberrations, it is preferable to place an aspheric surface for correction of distortion at a position as close to the object plane (mask) as possible. On this occasion, when the aspheric surface satisfies the foregoing condition (b-1) or (b-2), the distortion can be corrected well even with increase in the numerical aperture and with increase in the size of the image field.
For accomplishing the foregoing first or second object, a fourth projection optical system according to the present invention is a dioptric projection optical system for forming an image of a pattern on a first surface, onto a second surface by action of radiation-transmitting refractors, comprising four or more lenses having their respective refracting powers, wherein when four lenses are selected in order from the first surface of the lenses having their respective refracting powers, at least one surface of the four lenses is an aspheric surface, and wherein, where Ca is a local, principal curvature near a center of an optical axis of the aspheric surface and Cb is a local, principal curvature in the meridional direction of an extreme marginal region of a lens clear aperture diameter of the aspheric surface, the following condition holds if the aspheric surface has a negative refracting power:
Cb/Ca less than 0.45xe2x80x83xe2x80x83(c-1);
on the other hand, in the present invention, the following condition holds if the aspheric surface has a positive refracting power:
Cb/Ca greater than 2.3xe2x80x83xe2x80x83(c-2).
In the present invention, the local, principal curvatures Ca, Cb can be expressed by the above equations (b-4) and (b-5) as an example. When the aspheric surface satisfies the foregoing condition (c-1) or (c-2), the distortion can be corrected well even with increase in the numerical aperture and with increase in the size of the image field.
For accomplishing the foregoing third object, a fifth projection optical system according to the present invention is a projection optical system for forming a reduced image of a pattern on a first surface, onto a second surface, comprising: in the order named hereinafter from the first surface side, a first lens unit having a negative refracting power; a second lens unit having a positive refracting power; a third lens unit having a negative refracting power; a fourth lens unit having a positive refracting power; an aperture stop; and a fifth lens unit having a positive refracting power; wherein the following conditions are satisfied:
xe2x88x921.3 less than 1/xcex21 less than 0,
and
0.08 less than L1/L less than 0.17,
where xcex21 is a composite, lateral magnification of the first lens unit and the second lens unit, L1 is a distance from the first surface to a lens surface closest to the second surface in the second lens unit, and L is a distance from the first surface to the second surface.
A second fabrication method of a projection optical system according to the present invention is a method of fabricating a projection optical system for forming a reduced image of a pattern on a first surface, onto a second surface, comprising: a step of preparing a first lens unit having a negative refracting power; a step of preparing a second lens unit having a positive refracting power; a step of preparing a third lens unit having a negative refracting power; a step of preparing a fourth lens unit having a positive refracting power; a step of preparing an aperture stop; a step of preparing a fifth lens unit having a positive refracting power; and a step of locating the first lens unit, the second lens unit, the third lens unit, the fourth lens unit, the aperture stop, and the fifth lens unit in the order named from the first surface side; wherein, where xcex21 is a composite, lateral magnification of the first lens unit and the second lens unit, L1 a distance from the first surface to a lens surface closest to the second surface in the second lens unit, and L a distance from the first surface to the second surface, the first and second lens units are prepared so as to satisfy the following condition:
xe2x88x921.3 less than 1/xcex21 less than 0,
and the first and second lens units are located so as to satisfy the following condition:
0.08 less than L1/L less than 0.17.
For accomplishing the foregoing third object, a sixth projection optical system according to the present invention is a projection optical system for forming a reduced image of a pattern on a first surface, onto a second surface, comprising at least one light-transmitting refractor located in an optical path of the projection optical system, wherein the following condition is satisfied:
0.46 less than C/L less than 0.64,
where C is a total thickness along the optical axis of the radiation transmitting refractor located in the optical path of the projection optical system and L is a distance from the first surface to the second surface.
A third fabrication method of a projection optical system according to the present invention is a method of fabricating a projection optical system for forming a reduced image of a pattern on a first surface, onto a second surface, comprising a step of preparing a first lens unit having a negative refracting power, a step of preparing a second lens unit having a positive refracting power, a step of preparing a third lens unit having a negative refracting power, a step of preparing a fourth lens unit having a positive refracting power, a step of preparing an aperture stop, a step of preparing a fifth lens unit having a positive refracting power, and a step of locating the first lens unit, the second lens unit, the third lens unit, the fourth lens unit, the aperture stop, and the fifth lens unit in the order named from the first surface side, wherein the first lens unit to the fifth lens unit are prepared so as to satisfy the following condition:
0.46 less than C/L less than 0.64,
where C is a total thickness along the optical axis of a light-transmitting refractor located in an optical path of the projection optical system and L a distance from the first surface to the second surface.
For accomplishing the foregoing third object, a seventh projection optical system according to the present invention is a projection optical system for forming a reduced image of a pattern on a first surface, onto a second surface, comprising at least three lens surfaces of aspheric shape, wherein the following condition is satisfied:
0.15 less than Ea/E less than 0.7,
where E is the total number of members having their respective refracting powers among radiation-transmitting refractors in the projection optical system and Ea the total number of members each provided with a lens surface of aspheric shape.
A fourth fabrication method of a projection optical system according to the present invention is a method of fabricating a projection optical system for forming a reduced image of a pattern on a first surface, onto a second surface, comprising: a step of preparing light-transmitting members so that at least three surfaces of lens surfaces of the radiation-transmitting refractors are of aspheric shape and so that the following condition is satisfied:
xe2x80x830.15 less than Ea/E less than 0.7,
where E is the total number of members having their respective refracting powers among the radiation-transmitting refractors and Ea the total number of members each provided with a lens surface of aspheric shape; and a step of assembling the radiation transmitting members.
A first projection exposure apparatus according to present invention is a projection exposure apparatus for projecting a reduced image of a pattern provided on a projection master, onto a workpiece to effect exposure thereof, comprising: a light source for supplying exposure light; an illumination optical system for guiding the exposure light from the light source to the pattern on the projection master; and the projection optical system being either one selected from said projection systems; wherein the projection master can be placed on the first surface of the projection optical system, and the workpiece be placed on the second surface.
A second projection exposure apparatus according to the present invention is a projection exposure apparatus for projecting a reduced image of a pattern provided on a projection master, onto a workpiece to effect exposure thereof while scanning, comprising: a light source for supplying exposure light; an illumination optical system for guiding the exposure light from the light source to the pattern on the projection master; the projection optical system being either one selected from said projection optical systems; a first stage for enabling the projection master to be placed on the first surface of the projection optical system; and a second stage for enabling the workpiece to be placed on the second surface; wherein the first and second stages are movable at a ratio of speeds according to a projection magnification of the projection optical system.
For accomplishing the foregoing fourth object, a third projection exposure apparatus according to the present invention is a projection exposure apparatus for projecting a reduced image of a pattern provided on a projection master, onto a workpiece to effect exposure thereof, comprising: a light source for supplying exposure light in a wavelength region of not more than 180 nm; an illumination optical system for guiding the exposure light from the light source to the pattern on the projection master; and a projection optical system located in an optical path between the projection master and the workpiece, the projection optical system guiding 25% or more by quantity of the exposure light having passed through the projection master, to the workpiece to form the reduced image of the pattern on the workpiece.
A first projection exposure method according to the present invention is a projection exposure method of projecting a pattern formed on a projection master, onto a workpiece to effect exposure thereof, which uses the projection exposure apparatus being either one of the projection exposure apparatus, wherein the projection master is placed on the first surface and the workpiece is placed on the second surface, and wherein an image of the pattern is formed on the workpiece through the projection optical system.
A fourth projection exposure apparatus and a second projection exposure method according to the present invention are a projection exposure apparatus and a projection exposure method for projecting a reduced image of a pattern provided on a projection master, onto a workpiece to effect exposure thereof, which comprise: a light source for supplying exposure light in a wavelength region of not more than 200 nm; an illumination optical system for guiding the exposure light from the light source to the pattern on the projection master; and a projection optical system located in an optical path between the projection master and the workpiece, the projection optical system guiding the exposure light having passed through the projection master, to the workpiece to form the reduced image of the pattern on the workpiece; wherein the following condition is satisfied:
(En4/En3) greater than (En2/En1)
where En1 is a quantity of the exposure light traveling from the light source to the illumination optical system, En2 a quantity of the exposure light traveling from the illumination optical system to the projection master, En3 a quantity of the exposure light entering the projection optical system, and En4 a quantity of the exposure light emerging from the projection optical system toward the workpiece.
A first microdevice fabrication method according to the present invention is a method of fabricating a microdevice having a predetermined circuit pattern, comprising: a step of projecting an image of the pattern onto the workpiece to effect exposure thereof, using the foregoing exposure method; and a step of developing the workpiece after the projection exposure.
Next, for accomplishing the foregoing first or second object, a fifth projection exposure apparatus according to the present invention is a projection exposure apparatus for projecting a pattern on a projection master onto a workpiece to effect exposure thereof, comprising: an illumination optical system for supplying exposure light of a wavelength of not more than 200 nm to the projection master; and a projection optical system for forming an image of the pattern on the projection master, at a predetermined projection magnification xcex2 on the workpiece; wherein the projection optical system comprises an aperture stop, a front lens unit located between the aperture stop and the projection master, and a rear lens unit located between the aperture stop and the workpiece, wherein, where y (kg) represents a translated amount of fluorite of a disk member from an amount of fluorite among light-transmitting optical materials in the projection optical system, f2 (mm) represents a focal length of the rear lens unit, and NAw represents a maximum numerical aperture on the image side of the projection optical system, and where a parameter x is defined as is follows:
x=f2xc2x74|xcex2|xc2x7NAw2;
the following conditions are satisfied:
yxe2x89xa64xxe2x88x92200,
yxe2x89xa6(4x/13)+(1000/13),
yxe2x89xa74xxe2x88x92440,
and
yxe2x89xa70.
A sixth projection exposure apparatus according to the present invention is a scanning projection exposure apparatus for projecting a pattern on a projection master onto a workpiece to effect exposure thereof while scanning, comprising: an illumination optical system for supplying exposure light of a wavelength of not more than 200 nm to the projection master; and a projection optical system for forming an image of the pattern on the projection master, at a predetermined projection magnification xcex2 on the workpiece; wherein the projection optical system comprises an aperture stop, a front lens unit located between the aperture stop and the projection master, and a rear lens unit located between the aperture stop and the workpiece, wherein, where y (kg) represents a translated amount of fluorite of a disk member from an amount of fluorite among light-transmitting optical materials in the projection optical system, f2 (mm) a focal length of the rear lens unit, and NAw a maximum numerical aperture on the image side of the projection optical system, and where a parameter x is defined as follows:
x=f2xc2x74|xcex2|xc2x7NAw2;
the following conditions are satisfied:
yxe2x89xa64xxe2x88x92200,
yxe2x89xa6(4x/13)+(1000/13),
yxe2x89xa74xxe2x88x92440,
and
yxe2x89xa70.
A seventh projection exposure apparatus according to the present invention is a projection exposure apparatus for projecting a pattern on a projection master onto a workpiece to effect exposure thereof, comprising: an illumination optical system for supplying exposure light of a wavelength of not more than 200 nm to the projection master; and a projection optical system for forming an image of the pattern on the projection master, at a predetermined projection magnification xcex2 on the workpiece; wherein the projection optical system comprises an aperture stop, a front lens unit located between the aperture stop and the projection master, and a rear lens unit located between the aperture stop and the workpiece, wherein, where y (kg) represents a translated amount of fluorite of a disk member from an amount of fluorite among light-transmitting optical materials in the projection optical system, f2 (mm) a focal length of the rear lens unit, and NAw a maximum numerical aperture on the image side of the projection optical system, and where a parameter x is defined as follows:
x=f2xc2x74|xcex2|xc2x71 NAw2;
the following conditions are satisfied:
yxe2x89xa6(9x/2)xe2x88x92270,
yxe2x89xa690,
yxe2x89xa7(9x/2)xe2x88x92(855/2),
and
yxe2x89xa70.
An eighth projection exposure apparatus according to the present invention is a projection exposure apparatus for projecting a pattern on a projection master onto a workpiece to effect exposure thereof, comprising: an illumination optical system for supplying exposure light of a wavelength of not more than 200 nm to the projection master; and a projection optical system for forming an image of the pattern on the projection master, at a predetermined projection magnification xcex2 on the workpiece; wherein the projection optical system comprises an aperture stop, a front lens unit located between the aperture stop and the projection master, and a rear lens unit located between the aperture stop and the workpiece, wherein, where y (kg) represents a translated amount of a first material of a disk member from an amount of the first material among light-transmitting optical materials in the projection optical system, f2 (mm) represents a focal length of the rear lens unit, and NAw represents a maximum numerical aperture on the image side of the projection optical system, and where a parameter x is defined as follows:
x=f2xc2x74|xcex2|xc2x7NAw2;
the following conditions are satisfied:
yxe2x89xa64xxe2x88x92200,
yxe2x89xa6(4x/13)+(1000/13),
yxe2x89xa74xxe2x88x92440,
and
yxe2x89xa70.
Next, for accomplishing the foregoing first or second object, a third projection exposure method according to the present invention is a projection exposure method of projecting a pattern on a projection master onto a workpiece to effect exposure thereof, comprising: an illumination step of supplying exposure light of a wavelength of not more than 200 nm to the projection master; and an image forming step of forming an image of the pattern on the projection master, at a predetermined projection magnification xcex2 on the workpiece, using a projection optical system comprising a front lens unit, an aperture stop, and a rear lens unit; wherein the image forming step comprises a first auxiliary step of guiding the light from the projection master to the front lens unit, a second auxiliary step of guiding the light passing through the front lens unit, to the aperture stop, a third auxiliary step of guiding the light passing through the aperture stop, to the rear lens unit, and a fourth auxiliary step of forming the image of the pattern on the workpiece, using the light passing through the rear lens unit, wherein, where y (kg) represents a translated amount of fluorite of a disk member from an amount of fluorite among light-transmitting optical materials in the projection optical system, f2 (mm) represents a focal length of the rear lens unit, and NAw represents a maximum numerical aperture on the image side of the projection optical system, and where a parameter x is defined as follows:
x=f2xc2x74|xcex2|xc2x7NAw2;
the following conditions are satisfied:
yxe2x89xa64xxe2x88x92200,
yxe2x89xa6(4x/13)+(1000/13),
yxe2x89xa74xxe2x88x92440,
and
yxe2x89xa70.
A fourth projection exposure method according to the present invention is a projection exposure method of projecting a pattern on a projection master onto a workpiece to effect exposure thereof, comprising: an illumination step of supplying exposure light of a wavelength of not more than 200 nm to the projection master; and an image forming step of forming an image of the pattern on the projection master, at a predetermined projection magnification xcex2 on the workpiece, using a projection optical system comprising a front lens unit, an aperture stop, and a rear lens unit; wherein the image forming step comprises a first auxiliary step of guiding the light from the projection master to the front lens unit, a second auxiliary step of guiding the light passing through the front lens unit, to the aperture stop, a third auxiliary step of guiding the light passing through the aperture stop, to the rear lens unit, and a fourth auxiliary step of forming the image of the pattern on the workpiece, using the light passing through the rear lens unit, wherein, where y (kg) represents a translated amount of a first material of a disk member from an amount of the first material among light-transmitting optical materials in the projection optical system, f2 (mm) represents a focal length of the rear lens unit, and NAw represents a maximum numerical aperture on the image side of the projection optical system, and where a parameter x is defined as follows:
x=f2xc2x74|xcex2|xc2x7NAw2;
the following conditions are satisfied:
yxe2x89xa64xxe2x88x92200,
yxe2x89xa6(4x/13)+(1000/13),
yxe2x89xa74xxe2x88x92440,
and
yxe2x89xa70.
Next, a fabrication method of projection exposure apparatus according to the present invention is a method of fabricating the fifth, sixth, or seventh projection exposure apparatus of the present invention, comprising a step of preparing an illumination optical system for supplying exposure light of a wavelength of not more than 200 nm to the projection master; and a step of preparing a projection optical system for forming an image of the pattern on the projection master, at a predetermined projection magnification xcex2 on the workpiece; wherein the step of preparing the projection optical system comprises an auxiliary step of preparing a front lens unit, an aperture stop, and a rear lens unit, an auxiliary step of locating the front lens unit between positions where the aperture stop and the projection master are located respectively, and an auxiliary step of locating the rear lens unit between positions where the aperture stop and the workpiece are located respectively.
Next, a second microdevice fabrication method of the present invention is a method of fabricating a microdevice having a predetermined circuit pattern, comprising: a step of projecting an image of the pattern onto the workpiece to effect exposure thereof, using the third or fourth projection exposure method of the present invention; and a step of developing the workpiece after the projection exposure.
Next, for accomplishing the foregoing first or second object, an eighth projection optical system of the present invention is a dioptric projection optical system for forming an image of a pattern on a first surface, on a second surface, using light of a wavelength of not more than 200 nm, comprising: an aperture stop; a front lens unit located between the aperture stop and the first surface; and a rear lens unit located between the aperture stop and the second surface; wherein, where y (kg) represents a translated amount of fluorite of a disk member from an amount of fluorite among light-transmitting optical materials in the projection optical system, f2 (mm) a focal length of the rear lens unit, xcex2 a projection magnification of the projection optical system, and NAw a maximum numerical aperture on the image side of the projection optical system, and where a parameter x is defined as follows:
x=f2xc2x74|xcex2|xc2x7NAw2;
the following conditions are satisfied:
yxe2x89xa64xxe2x88x92200,
yxe2x89xa6(4x/13)+(1000/13),
yxe2x89xa74xxe2x88x92440,
and
yxe2x89xa70.
A ninth projection optical system according to the present invention is a dioptric projection optical system for forming an image of a pattern of a first surface on a second surface, using light of a wavelength of not more than 200 nm, comprising: an aperture stop; a front lens unit located between the aperture stop and the first surface; and a rear lens unit located between the aperture stop and the second surface; wherein, where y (kg) represents a translated amount of a first material of a disk member from an amount of the first material among light-transmitting optical materials in the projection optical system, f2 (mm) represents a focal length of the rear lens unit, xcex2 a projection magnification of the projection optical system, and NAw represents a maximum numerical aperture on the image side of the projection optical system, and where a parameter x is defined as follows:
x=f2xc2x74|xcex2|xc2x7NAw2;
the following conditions are satisfied:
yxe2x89xa64xxe2x88x92200,
yxe2x89xa6(4x/13)+(1000/13),
yxe2x89xa74xxe2x88x92440,
and
yxe2x89xa70.
Fifth and sixth fabrication methods of a projection optical system according to the present invention are methods of fabricating the eighth and ninth projection optical systems, respectively, of the present invention, which comprise a step of preparing a front lens unit, an aperture stop, and a rear lens unit, a step of locating the front lens unit between the aperture stop and the first surface, and a step of locating the rear lens unit between the aperture stop and the second surface.